Clearly this writer had pounds invested in girl (7)
I believe the answer is:
lucidly
'clearly' is the definition.
(thesaurus)
'this writer had pounds invested in girl' is the wordplay.
'this writer' becomes 'i' (the person who is writing these clues).
'had' becomes 'd' (I've seen this in other clues).
'pounds' becomes 'l' (L can represent the monetary pound).
'invested in' means one lot of letters goes inside another.
'girl' becomes 'lucy' (**).
'i'+'d'+'l'='idl'
'idl' put within 'lucy' is 'LUCIDLY'.
(Another definition for lucidly that I've seen is " Clearly, allowing understanding".)